(Note: The background section of this page uses most of the same words as Terra Futura's page on turbofans to save time.)
Early airplanes used piston engines similar to those used in cars, buses, and trains. As planes became common in the 1930s, radial engines started being used. In the early years of air travel, the radial engine was the dominant aircraft engine. The radial engine was used to turn a propeller which helped the airplane fly. After World War II, gas turbine engines started being used for propulsion. The earliest were turboprop engines which continued to use a propeller. In more large scale aircraft, the turboprop was initially replaced with the turbojet. But turbojets were inefficient and noisy. So they were replaced with turbofans.
Turbofans became commonplace after they were first used on the Boeing 707. The jet age was born. This was followed by the 727. Early turbofans were low bypass. The fan had multiple stages. It could go faster. In the 1960s, turbofans became high bypass. There was only one fan. They more efficient than ever before. By the early 21st century, the fastest turbofan used for passenger travel was the Boeing 787. It reached Mach .89 once it finally got in the air. In the 21st century, turbofans were mainly used for subsonic travel. Other engines went supersonic.
Tech Level: 10-11
Precooled Jet Engines evolved from the concept of liquid air cycle engines of the 1960s. Unlike them, however, precooled jet engines did not liquify the air. They just heated it. Fueled by liquid hydrogen, the only waste products were water vapor and laughing gas. The most famous use of precooled jet engines was in the LAPCAT A2. The Scimitar engines used on the A2 were precooled air turboramjets that traveled at Mach 5. Later designs, of course, would travel even faster. This was the shape of engines to come.